The advent of Ethernet as a metropolitan-area networks (MANs) and wide area networks technology imposes a new set of Ethernet layer operation, administration, and management (OAM) requirements on Ethernet's traditional operations, which were centered on enterprise networks only. The expansion of Ethernet technology into the domain of service providers, where networks are substantially larger and more complex than enterprise networks and the user base is wider, makes operational management of link uptime crucial. More importantly, the timeliness in isolating and responding to a failure becomes mandatory for normal day-to-day operations, and OAM translates directly to the competitiveness of the service provider.
Ethernet Connectivity Fault Management (CFM or Ethernet CFM) is an end-to-end per-service-instance OAM protocol that provides proactive connectivity monitoring, fault verification, and fault isolation for large Ethernet MANs and WANs. CFM uses standard Ethernet frames. ALL CFM frames are distinguishable by EtherType and in case of multicast frames by Media Access Control (MAC) addresses. CFM frames are sourced, terminated, processed, and relayed by bridges. Bridges that cannot interpret CFM messages forward them as normal data frames. Ethernet CFM comprises three protocols that work together to help administrators debug Ethernet networks. These are: continuity check, link trace and loopback protocols.
CFM continuity check messages are “heartbeat” messages issued periodically by MEPs. They allow MEPS to detect loss of service connectivity amongst themselves. They also allow MEPs to discover other MEPs within a domain, and allow MIPs to discover MEPs. CFM link trace messages are transmitted by a MEP on the request of the administrator to track the path (hop-by-hop) to a destination MEP. CFM link trace messages allow the transmitting node to discover vital connectivity data about the path. CFM loopback messages are transmitted by a MEP on the request of the administrator to verify connectivity to a particular maintenance point. Loopback indicates whether the destination is reachable or not; it does not allow hop-by-hop discovery of the path. It is similar in concept to the concept of a “ping” message.